(1) Field of the Invention
The present invention relates to a solid-type thermoelectric transducer that achieves cooling operation by using thermoelectrons and field emission electrons. In particular, the present invention relates to a thermoelectric transducer comprising an electron transport layer having a porous structure in which a solid phase, composed of fine particles of an electrically non-conductive insulating material, and a vapor phase coexist, a manufacturing method thereof, a cooling device using the thermoelectric transducer, and a method for controlling the cooling device.
(2) Description of the Related Art
Thermoelectric transducers that achieve cooling operation by emitting electrons by using emitter materials that are able to easily emit electrons due to the action of heat or an electric field are disclosed in, for example, Journal of Applied Physics, Vol. 76, No. 7 (1994), page 4362, (hereunder referred to as Document 1); Applied Physics Letters, Vol. 78, No. 17 (2001), page 2572, (hereunder referred to as Document 2); and U.S. Pat. No. 5,675,972 (hereunder referred to as Document 3).
FIG. 4(a) is a cross-sectional view showing the basic structure of the prior art thermoelectric transducers disclosed in the above-mentioned documents 1 to 3 (prior art example 1). The basic operating principle of a thermoelectric transducer is explained with reference to FIG. 4(a).
The thermoelectric transducer shown in FIG. 4(a) comprises an emitter 1 that is connected to an object to be cooled (not shown) in such a manner that heat is exchangeable between the emitter 1 and the object to be cooled (hereunder referred to as “thermally connected”), a collector 2 thermally connected to an object to be heated (not shown), and a power supply 4 for applying voltage across these electrodes. The emitter 1 and the collector 2 are arranged so as to oppose each other having a fine gap formed by using spacers 11, etc., in between, wherein the gap between the emitter 1 and the collector 2 is a vacuum space 10 (vapor phase).
If a positive voltage is applied to the collector 2 of this element and a negative voltage is applied to the emitter 1 when the surface of the emitter 1 is in a condition that allows electrons to be easily emitted in the vacuum space 10, i.e., when it has a low work function, electrons 5 will be emitted by thermal action and/or electric-field action at a certain threshold or above. The emitted electrons 5 travel from the emitter 1 to the collector 2 using the vacuum space 10 as an electron transfer path.
In this case, the electrons 5 emitted from the emitter 1 are taken into the collector 2 retaining the energy that they held when they were inside the emitter 1. In other words, by making electrons 5 travel between the emitter 1 and the collector 2 through the vacuum fine gap, it becomes possible to transfer the heat of the emitter 1 to the collector 2. Therefore, the emitter 1 and the object thermally connected to the emitter 1 are cooled. On the other hand, the collector 2 to which the electrons 5 holding energy are supplied and the object that is thermally connected to the collector 2 are heated.
The above operation can be summarized as follows: By applying a voltage to the thermoelectric transducer and causing electrons to be emitted from the emitter 1, the periphery of the emitter 1 is cooled by endothermic action and the periphery of the collector 2 is heated by heat dissipation.
To operate such a thermoelectric transducer efficiently, a means for easily emitting the electrons 5 is important, and, for that purpose, the formation of an emitter material with a low work function and the formation of the fine gap structure are necessary.
Furthermore, another structure of a thermoelectric transducer using the same principle is disclosed in U.S. Pat. No. 4,019,113 (hereafter referred to as Document 4). FIG. 4(b) shows the structure of the thermoelectric transducer disclosed in Document 4 (prior art example 2). This thermoelectric transducer is a solid-type that does not use a vacuum space (vapor phase) as an electron transfer path but uses a thin film 12 (solid phase). Note that, in FIG. 4(b), the same symbols are given to the same constituents as in the thermoelectric transducer (prior art example 1) shown in FIG. 4(a).
Also in this case, the operating principle is the same as that in the above-mentioned structure; however, it differs from prior art example 1 in that it uses the thin film 12 (solid phase) as the space to which electrons are emitted to enhance the efficiency of the electron emission.
Furthermore, advanced examples of this solid-type thermoelectric transducer are disclosed in U.S. Pat. No. 6,489,704 (hereafter referred to as Document 5), and national publication of the translated version of PCT application No. 2002-540636 (hereafter referred to as Document 6). The schematic structure of these solid-type thermoelectric transducers is shown in FIG. 4(c) (prior art example 3). Also in this figure, the same symbols are given to the same constituents as in the thermoelectric transducer (prior art example 1) shown in FIG. 4(a). In the thermoelectric transducer of prior art example 3, unlike the thermoelectric transducer of prior art examples 1 and 2, a performance improvement is attempted by separating a solid phase 13, which serves as an electron transfer path, and a vapor phase 14.
A cooling device using these thermoelectric transducers is characterized in that it does not have any moving elements and is smaller than conventional mechanical compressing devices, and cooling mediums, such as chlorofluocarbon, are unnecessary. Furthermore, since the theoretical cooling efficiency is also high, it is considered to be an ideal cooling device.
However, in the thermoelectric transducer of prior art example 1 disclosed in Document 3, it is necessary to form a fine gap for vacuum space 10 as shown in FIG. 4(a). Therefore, to maintain a stable thermionic transfer characteristic, it is necessary to form a very narrow space (generally about 10 to 500 nm) with high accuracy by using spacers 11 or the like, and at the same time, the space has to maintain a high vacuum. In other words, in the thermoelectric transducer of the prior art structure, it is difficult to produce a very narrow vacuum gap in a large area with sufficient accuracy.
Moreover, in the thermoelectric transducer of prior art example 2 disclosed in Document 4, although some of the problems of prior art example 1 are solved by changing the electron transfer region from a vacuum (vapor phase) to a thin film (solid phase) as shown in FIG. 4(b), because the emitter 1 to be cooled and the collector 2 to be heated contact each other through a solid-phase region (thin film 12), the electron transfer region will be significantly affected by the heat conduction from a hot part (collector 2) to a cold part (emitter 1). In other words, to maintain an efficient thermoelectric transfer characteristic, it is desirable to prevent an outflow of heat from the hot part to the cold part as much as possible; however, because the electron transfer region is a laminated structure of thin films in prior art example 2, there is a large loss (runoff of heat) due to heat conduction from the hot part to the cold part.
Furthermore, in the thermoelectric transducer of prior art example 3 disclosed in Document 5, some of the problems of prior art example 2 are solved by spatially separating the electron transfer region, which is composed of a solid phase 13, and the heat-conduction suppression region, which is composed of a vapor phase 14, as shown in FIG. 4(c), but energy loss is large because it is structured so that electrons are implanted into the solid phase 13 through a fine contact. In other words, although the thermoelectric transducer of prior art example 3 provides effective fine gap formation and prevention of heat-conduction, its efficiency is insufficient because it conducts electrons in the solid phase 13 through a fine contact.